Problem: Simplify the following expression: $k = \dfrac{20b}{25b - 20c} - \dfrac{20b - 20c}{25b - 20c}$ You can assume $a,b,c \neq 0$.
Answer: Since the expressions have the same denominator we simply combine the numerators: $k = \dfrac{20b - (20b - 20c)}{25b - 20c}$ $k = \dfrac{20c}{25b - 20c}$ The numerator and denominator have a common factor of $5$, so we can simplify $k = \dfrac{4c}{5b - 4c}$